1. Field of the Invention
The present invention relates to the measurement of the polarization of light and particularly to the encoding of measured light polarization characteristics within a two dimension spectrographic array.
2. Description of the Background
Electromagnetic radiation in the form of light is characterized by various properties including intensity, direction of propagation, frequency or wavelength spectrum, and polarization. Analysis of the intensity and spectral characteristics of light is a common diagnostic approach for deducing related properties of the light's source, as well as reflection and transmission mediums between the source and the receiver. Collection and analysis of spectrographic information is a cornerstone approach to astrophysics and has applications in numerous other disciplines such as materials science, remote sensing, medical diagnostics, defense, biophysics, microscopy and fundamental physics. Most astronomical spectrographs use a telescope to focus light from an astronomical source onto a slit. Light from the slit is passed to a collimator which turns the diverging light beam into parallel light, and then to a disperser (usually a reflection grating) to create a spectrum, and then to a camera that focuses the spectrum onto a sensor such as a CCD. The horizontal axis of the spectral image no longer corresponds to a spatial direction in the sky, but rather now represents wavelength. The vertical axis of the image still corresponds to a spatial position of the incident light source. The result is a two-dimensional, spatially resolved spectrograph image comprising a band of varying intensity stretching across the image in the spectral direction and illustrating intensity as a function of wavelength. The image contains several spectra, each corresponding to a different position in the slit, or more precisely, a different part of the source along the slit.
Polarization is a property of light waves that describes the orientation of their oscillations. “Spectropolarimetry” is the measurement of the polarization of light that has been dispersed into a continuum or line spectrum as a function of wavelength. Spectropolarimetry provides a versatile suite of diagnostics tools. For example, in astrophysical research polarimetry can be used to deduce the properties of astrophysical dust when that dust scatters the light of a nearby star. The starlight becomes polarized when scattered by the dust in a way that depends on the size, porosity and composition of the dust particles. It is possible to measure these characteristics using polarimetry information of the scattered light. Consequently, scientific research based on spectropolarimetry techniques is undergoing a phase of rapid growth, especially in astronomy where spectropolarimetric observations are providing important clues as to planets, stars and the origins of the universe.
The polarization characteristic of light can described by the Stokes vector (I, Q, U, V) in which I is the total intensity, Q and U yield the linear polarization in each of two planes at 45 degrees to one another, perpendicular to the direction of wave propagation and, and V is the circular polarization. Normalized stokes polarization parameters (q, u, and v) represent the fractional polarization state (Q/I, U/I and V/I, respectively). Traditionally polarization measurements are made sequentially with polarizing filters at different orientations, with rotatable wave plates, or with complex, fragile, rapidly modulating components such as ferroelectric liquid crystals and resonant crystal photoelastic modulators (PEMs) to achieve high precision. However, sequential measurement entails moving parts, and modulating components typically lead to inherently monochromatic performance and component fragility, all of which introduce mechanical complexity, potential for error, and generally decrease the utility of polarimeters. Indeed, polarimeters employing such schemes are either too imprecise because of the need for sequential measurement, or too impractical for reliable deployment in a space based astronomical observatory.
The general concept of a “point-and-shoot” polarimeter capable of taking polarization measurements in real time is well-known. For example, United States Patent Application 20050007591 by Shribak et al. (Marine Biological Laboratory) shows a point-and-shoot polarization measurement system and method for a sampling spectrometer. The invention splits a light beam into several beams, which are analyzed using elliptical polarizers and the resultant intensity is measured. United States Patent Application 20100271475 to Schwiegerling et al. also shows a point-and-shoot imaging polarimeter. This Schwiegerling device is capable of acquiring all four components of the Stokes Vectors from an existing image without any moving parts. Rather, two Savart Plates are used in that device to generate four relatively shear beams to interfere at the imaging plane. United States Patent Application PG Publications 2006/0238759 and 2011/0080586 by Okabe et al. show a method of spectroscopic polarimetry in which two retarders are used to create a frequency dependent phase difference between the orthogonal polarization components of light under measurement. Amplitude and a phase of each of the carrier components are modulated by the Stokes Parameters of the light under measurement. It therefore becomes possible to obtain each of the Stokes Parameters by execution of signal processing with a computer by use of Fourier transformation. However, the method disclosed by Okabe et al. is designed for measuring polarization characteristics of a sample by decoding the polarization information using only Fourier analysis of the spectral amplitude variations. That is, the carrier amplitude variations are encoded along the same dimension as the wavelength amplitude variations (the spectrum). Neither it nor the foregoing references incorporate polarimetry in a conventional long-slit spectrograph which is more suited for astronomical spectrographs. With astronomical spectrographs, the scalar values of the digital image are typically replaced with scalar values yielding intensity at a location along one dimension, and wavelength (one spatial dimension and one spectral dimension). In this context spectroscopic polarimetry is more complex. The polarization characteristics must be encoded directly into the spectrum which is imaged onto a conventional detector, such as a CCD or CMOS, so that said characteristics can be derived by a processor. Okabe et al. modulate amplitude and phase of each of the carrier components by the Stokes Parameters of the light under measurement so that each of the Stokes Parameters can be derived by execution of signal processing with a computer by use of Fourier transformation. Along these same lines, A. M. Locke et al., “Snapshot Imaging Spectropolarimeter”, Optical Sciences Center, University of Arizona, Tuscon, Ariz. 85721, discusses “channeled spectropolarimetry” which also uses amplitude modulation to encode the spectral dependence of all four Stokes parameters into a single spectrum, along with a Computed Tomography Imaging Spectrometer to provide imaging information. The spectropolarimetry data is represented as an image of a four-dimensional volume: two spatial variables (x, y), wavenumber (σ), and the Stokes vector index (j). The Stokes vector index has only four possible values (the integers from 0 to 3).
Snik et al., “HARPSpol—The New Polarimetric Mode for HARPS”, Telescopes and Instrumentation (2009) which discusses the HARPS polarimeter. The HARPS polarimeter uses two optical fibers to split the collected light from the Cassegrain reflector into two orthogonal polarizations allowing sensitive and accurate measurements of both circular and linear polarisations of stellar light as a function of wavelength at high spectral resolution. The HARPS spectrograph shows both circular and linear polarizations for practically every spectral line.
J. H. H. Rietjens et al, SPEX: The Spectropolarimeter For Planetary Exploration, International Conference on Space Optics (2010) describes the use of multiple retarders and a beam slitter to modulate the radiance spectrum in both amplitude and phase by the degree and angle of linear polarization, respectively. The technique encodes the degree of linear polarization and angle of linear polarization of the incident light in the measured flux spectra.
The foregoing references form a spectrally dispersed image of the slit on a two-dimensional detector array. Thus one (spectral) dimension on the detector array (perpendicular to the entrance slit) corresponds to wavelength while the other spatial dimension corresponds to spatial position along the slit. The references encode the polarization information along the spectral dimension (the direction of dispersion perpendicular to the slit), not the spatial dimension (along the slit).
It would be more advantageous to encode the polarization information directly onto the spectrograph imaged on the spatial dimension of the detector/CCD itself so that a single acquired image combines spatial, spectral, and polarimetric information allowing it to be analyzed in both the spectral dimension and polarimetric dimension. The present inventor accomplishes this by oversampling and expanding the spectrograph to show full-or-partial Stokes polarimetry data along the spatial direction (orthogonal to the slit).